This invention relates to the wafer handling, and particularly to the transfer and holding, of semiconductor wafer substrates during semiconductor manufacture, more particularly, wafers of diameters of 200 millimeters (mm) and larger.
In the semiconductor industry, many companies manufacture equipment to process semiconductor wafers, particularly silicon wafers, for device production. Semiconductor wafer processing equipment employs automated and robotic wafer handlers for moving the wafers through the processing equipment and for holding the wafers for processing. Wafer handlers in the industry typically involve the use of a vacuum chuck that includes a vacuum-type spatula or end effector on a robotic arm, which makes contact with the backside of the wafer. With the more versatile handlers, contact with the wafer is made by the vacuum chuck in a circular area at the center of the wafer. One such wafer handling system is described in U.S. Pat. No. 5,820,329, hereby expressly incorporated by reference herein. Such wafer handling is typical in many wafer processing machines for the processing of the device sides of the wafers.
In semiconductor manufacture, when processing of the device side of a wafer is complete, a backside metallization layer is sometimes applied. For some devices, the metallization layer is often gold. Backside metallization with gold, and wafer holders for such processes, is described in the commonly assigned U.S. Pat. No. 6,258,228, filed Jan. 8, 1999, and hereby expressly incorporated by reference herein. For backside metallization, similar processing equipment is used as is used to process the device sides of wafers, but the wafer orientation is reversed. Reversal of the orientation of a partially processed wafer would expose the devices at the center of the wafer to contact by the vacuum chuck of the wafer transfer arm. The devices usually cannot be subjected to such contact without suffering damage.
For this reason, vacuum chucks have been developed to grip the wafer along a 6 mm ring inside of the edge of the wafer. As a result, a 6 mm ring at the edge of the wafer is reserved as an exclusion zone in which the wafer cannot be used for device manufacture. The 6 mm ring of exclusion is needed to provide a surface area that is enough to enable a vacuum chuck to reliably hold the wafer in the vertical, horizontal and inverted orientations that are required of a handler. Typically, for a wafer of 200 mm in diameter, a 6 mm contact area on the wafer engaged by elements of the wafer handler along the edge of the wafer, amounts to an area of over 36 square centimeters or twelve percent of the area of the wafer. A need has been expressed in the industry for the contact areas between the wafers and the wafer handlers to be reduced, preferably to not more than two mm around the edge of a 200 mm or 300 mm wafer. A two mm exclusion zone contains an area of only about 12 square centimeters on a 200 mm wafer and 18 square centimeters on a 300 mm wafer. This need has not been filled in the prior art.
Wafer handlers operate and are controlled in conjunction with the operation and control of the machines of which they are a part or with which they interact. Fundamental changes in the nature and operation of wafer handlers, if made, may be incompatible with, and can adversely affect, the operation and control of the semiconductor processing machines. Unless wafer handler changes are accompanied by replacement or redesign of the machines (e.g. via a kit), impact on operating software and on system operation can occur.
These contact areas typically prevent use of the portion of the wafer bounded by the contacted area for device manufacture, limiting the per-wafer device yield. As pattern geometries become smaller and demands for higher per-wafer yield become greater, the need for increased useful area of the wafer becomes greater.
Accordingly, there is a need for a wafer handler and a wafer handling technique that provides for a smaller contact area or exclusion zone where contact with the wafer is allowed. There is also a need for such improved wafer handling in a way that does not impact upon the operating software and systems operation of the machines with which such an improved handler or handling technique is used.
A primary objective of the present invention is to provide for the engaging and transfer of a semiconductor wafer for backside processing while making minimal contact with the device side of the wafer. A particular objective of the invention is to provide for the engagement and transfer of a semiconductor wafer while contacting the wafer in a zone of exclusion adjacent the periphery of the wafer, and more particularly, where the zone of exclusion is not more than two mm wide.
A further objective of the present invention is to provide a method and apparatus for the holding and transfer of wafers that is capable of holding and transferring wafers of differing sizes, particularly of diameters of 200 mm and 300 mm, and that is capable among wafers of differing sizes.
Another objective of the invention is to provide a wafer holding and transferring system that provides for reduced device side contact for the backside processing of wafers that is compatible with, and can be retrofitted to, processing machines of the prior art that do not otherwise have such capability. A more particular objective of the invention is to provide for the retrofit of such a system to an existing processing machine with minimal impact on the hardware of the processing machine and with little or no impact on the operating software of the processing machine.
According to the principles of the present invention, a wafer handling system is provided having a wafer transfer arm chuck, a centering station chuck and a load arm chuck which can hold and exchange wafers between chucks, wafer cassettes and processing machine wafer holders while contacting a reduced area on the surface of the wafer, and which is capable of contacting only an exclusion zone of preferably not more than approximately two mm in width adjacent the periphery of the wafer on the device side of the wafer during backside processing.
In certain embodiments, the wafer chucks of the transfer arm and load arm have beveled edge rings or edge ring segments that are of a diameter larger than the diameter of the wafer. The beveled edge rings insure that only a narrow exclusion zone on the edge of the wafer comes into contact with surfaces on the chucks that support the wafer. For some applications, the centering station chuck may also be provided with such a beveled edge ring.
In certain embodiments, an end effector chuck is provided on the transfer arm. The end effector is of multiple piece construction to achieve greater flatness, uses an internal vacuum channel to determine wafer presence and has ceramic outriggers that are adjustable to accommodate wafers of differing diameter, for example, diameters of 200 mm and 300 mm.
The centering station chuck corrects wafer flat and crystal orientation of the wafer and wafer centering. The chuck is preferably of multiple piece construction and uses a vacuum channel to sense the presence of the wafer and optical sensors to sense the wafer flat orientation as the chuck rotates the wafer. The surface of the centering station chuck has recesses such as clearance grooves to allow the outriggers of the end effector to successfully place or remove a wafer within an acceptable window of the chuck orientation relative to a home position.
The load arm chuck is in some respects similar to the centering station chuck in the way that it interacts with the transfer arm chuck but does not rotate to alter the orientation of the wafer. It uses a vacuum channel to determine wafer presence. The load arm has pivotal wafer edge hooks or gripper elements that grip the edge of the wafer within the exclusion zone. The gripper elements are pneumatically operated by electro-pneumatic actuators that can be responsive to the same electrical control signals that operate the vacuum controls as were the vacuum chucks of previous load arms. The gripper elements of the load arm chucks are actuated in response to the motion of the robot transfer arm in relation to the load arm or the load arm in relation to the wafer holder of the processing machine. The gripper elements may be pivotal gripper hooks, or in lieu of the pivotal gripper hooks, may be other types of wafer holding devices such rotary latches, similar to those having pivotal rollers or non-contact tabs of the described prior art, which can be rotated over the edge of the wafer to latch the wafer to the load arm chuck. The gripper elements allow the load arm to hold the wafer in a vertical orientation or facing downwardly and while being held in or moved through some other or different orientations.
In certain embodiments of the invention, a vacuum chuck equipped system is retrofitted with mechanical wafer engaging chucks, particularly replacing the vacuum chucks on a transfer arm end effector, a wafer centering or aligning station and a wafer load arm. In such a system, the transfer arm may be operable to pick up and move horizontally disposed wafers, transferring them to and from wafer cassettes, the centering or aligning station and the load arm. Also, the chuck at the centering and aligning station may be operable to receive a wafer from the transfer arm, to orient and align the wafer and to return the wafer to a centered and oriented position on the transfer arm, also in a horizontal orientation. Additionally, the load arm chucks may be operable to move a wafer between the transfer arm chuck and a wafer holder of the wafer processing machine, or exchange one wafer with another, reorienting the wafers between a horizontal orientation on the transfer arm and a vertical orientation in the wafer holder of the processing machine. Wafers are held at least in part by gravity on the horizontally disposed, upwardly facing chucks of the transfer arm and aligning station and are held in part by gravity on the load arm chucks when they are horizontally disposed and upwardly facing during transfer of the wafers to and from the transfer arm, and by positive wafer edge gripper hooks, when being moved in other orientations.
In certain embodiments of the invention, the controls of the wafer handling system are compatible with the controls of the vacuum chuck equipped, prior art machine so that the system can be retrofitted thereto without substantial hardware changes and without modification to the control software of the machine. Vacuum chuck control lines are used to sense the presence of wafers on the chucks. Gripper operating pneumatic cylinders are operated by electro-pneumatic solenoids that are controlled by electrical software vacuum command signals that were provided for vacuum chuck operation.
Embodiments of the present invention may be provided in the form of a retrofit kit that includes the three chuck assemblies configured to replace vacuum chucks of the transfer arm, aligning station and load arm of existing processing machines.
The method and apparatus of the present invention provides the advantage of increasing the useful area of a wafer by approximately four percent, or from 88% to 96% of the area of the wafer, reducing by two-thirds the exclusion zone or unusable area of the wafer, and providing, on average, similar increases in the number of devices produced per wafer, thereby improving the productivity of the semiconductor making processes and machinery.
These and other objectives and advantages of the present invention will be more readily apparent from the following detailed description.